As a Star Becomes a Giant, Its Outer Layers Are

As a star becomes a giant, its outer layers are expanding. Where does the energy for expanding these layers come from?

A) from the fusion of helium into carbon in the core
B) from the long-term fusion of hydrogen into helium in the core
C) from an explosion in the core
D) from a magnetic dynamo effect in the star's outer layers, caused by a much stronger magnetic field inside the star
E) from the fusion of hydrogen into helium in a shell around the core